1. Field of the Invention
The present invention relates to a blow-molding method for resinous molding products, and more particularly, a blow-molding method for obtaining a resinous molding product in which specific portions thereof are made thick with respect to the circumferential direction.
2. Description of the Prior Art
Conventionally, the so-called blow-molding method has been generally known as one molding methods for efficiently manufacturing hollow resinous molding products (refer to, for example, Japanese Patent laid open Publication Sho No. 58-23212).
This blow-molding method is arranged to preliminarily mold a thermoplastic resinous material, for example, in a tube shape by extrusion or injection. Compressed air is blown into the inside of the preliminary molded member (parison) in the softened state while placing the parison between metallic molds so as to expand the parison along the molding surface of the metallic molds. The parison is then cooled to solidify the parison, resulting in molded hollow resinous molding products. This process can obtain molding products of stable quality at a high productivity, particularly in the mass production.
Conventionally, the above blow-molding method has been generally employed for molding containers particularly not requiring high strength, for example, containers such as fuel tanks in products for vehicles such as automobiles. In case of such containers, it is desirable to mold in such a way that the thickness of each part becomes as uniform as possible. Various devices and improvements have been made to meet this requirement.
On the other hand, in the field of the so-called structural components where some extent of strength is required, for example, seat frame members for vehicle use, employment of resinous moldings by the blow-molding has been recently tried to achieve further reduction in weight and cost in place of conventional metallic parts (for example, Japanese Patent laid open Publication Sho No. 62-48488).
In case of manufacturing the seat frame by blow-molding, since the seat frame 73 generally has a flat sectional surface as shown in FIG. 40, when a parison 70 extruded in a tubular shape as shown in FIG. 38 is put into the metallic mold as it is and expand-molded by compressed air, it is difficult to cause said parison to closely contact the inner face of the molding die, resulting in failure to obtain sufficient accuracy of molding.
In order to solve such a drawback, there has been tried a process so arranged that a pair of expander pins 71, are put into a cylindrical parison 70 as shown in FIG. 39 as a preliminary step before putting into the molding die. An extended parison 72 is thereby formed through moving said pins in the parting directions (directions indicated by arrows). Thereafter a predetermined blow-molding is effected with a molding die through employment of said expanded parison 72 having a shape close to the product shape.
However, in the case of extending the parison 70 with the extender pins 71 as a preliminary step of the original blow-molding, since the fluidity of said parison 70 is made approximately constant in the whole range, both end portions 72a which are in direct contact with the extender pins 71, 71 so as to directly undergo the extending force is the largest in the extension amount as shown in FIG. 39 and thus, become thin, while the central portions 72b, 72b located between said end portions become thick because of a comparatively small extension amount. Therefore, in the case of forming a seat frame with an extended parison having such a thickness difference by blow-molding, the back supporting portions 73b, 73b located centrally in the width direction are formed thick, while both end portions 73a, 73a are formed thin. In addition, in a seat frame having bank portions largely projecting from the back support portions 73b at both ends, this thinning of the thickness is further facilitated.
On the other hand, generally in the seat frame, since hinge mechanisms are provided on both ends through which to connect the seat frame to the body, both end portions are required to be high in rigidity, while the central portion of the seat frame acting as the back supporting portion is not required to be as rigid as both end portions. From this view-point, it may be said to meet the requirement for strength and weight reduction to form the seat frame to be thick in both end portions and thinner in the central portion.
However, by the conventional method, both end portions 73a which should be formed thick are formed thinner, while the back supporting portion 73b to be formed thin is formed thick. This results in the opposite characteristic to the strength characteristic required for the seat frame.
Furthermore, when the thickness of both end portions 73a are increased to secure the connection strength to the body of such a seat frame, the thickness of the back supporting portion 73b is further increased as a result of said thickness increase. Although the connection strength is secured consequently, this is not desirable because the weight reduction which is the merit of using a resinous material is impaired.
In other words, in this case, it is necessary to increase the thickness only in the both end portions supposed to be subject to a load more than predetermined during use as compared with the central portion acting as the back supporting portion and thereby, to secure the required strength on these specified portions while possibly suppressing the weight increase on the whole.
FIG. 8 is a perspective view of a resinous seat-back frame 25 for automobiles. As shown in FIG. 8, the thickness of both end portions 25a, 25b of the frame 25 is required to be set thicker as compared with that of the central portion 25c.
In order to form such a molded product by a blow-molding method while suppressing the weight increase on the whole, it is required to form a parison in thickness variation in the circumferential direction so that the portions corresponding to the both end portions 25a, 25b of the seat-back frame 25 may become thicker as compared with the other portion (central portion 25c).
In obtaining such a parison varied in thickness so that specified portions are thicker as compared with the other portion, die shaving or core shaving has been conventionally effected on the accumulator head of a parison extruder.
For example, description will be given below, taking the case of core shaving as an example. As shown in FIG. 41, the parison is arranged to be downward extruded from an approximately ring-shaped gap formed between a core 84 having a vertical axial line and a ring-shaped die 83 enclosing said core 84. On the outer peripheral surface of said core 84, shaving is made on the area in which the thickness of the parison is to be made thicker so as to make the gap 85a of said portion larger as compared with other portions.
Therefore, the parison becomes thicker at the portion corresponding to said gap 85a. When blow-molding is conducted onto such a parison, this portion is formed thicker with higher strength as compared with other portion. This secures the required strength on the specified portion of the molding.
However, when the parison is outward extruded from the accumulator head 81 provided with the shaving as described above, since there is a difference in the parison extruding speed between the gap portion 86a where core shaving is effected and the other gap portions 86, when the degree of thickness variation exceeds a predetermined limit, a defect such as the undulated portion 87c may arise on the surface of the parison 87 as shown in FIG. 42 or the parison 87 may be extruded in a bent state.
For this reason, the thickness variation ratio, that is, the ratio of the thickness of the thicker portion to that of the thinner portion is limited to a predetermined range (generally about 1.2) and it is very difficult to obtain a thickness ratio larger than that.
Regarding this problem, the applicant of the present invention has developed the following blow-molding method. Namely, the molding method is arranged so that, as shown in FIG. 43, a parison 97 is extruded preliminarily in such a varied thickness state that thick portions 97a and thin portions 97b are formed in the circumferential direction. A pair of expander members 98 which are movable outward in contact with the thick portions 97a are provided in the inner side of the parison 97. The thick portions 97a are brought into contact with the corresponding molding surfaces of the molding die 93 by moving respective expander members 98 towards the outside of said parison 97 as shown in FIG. 44, thus extending said thin portions 97b. Air is then blown into the parison 97 as shown in FIG. 45.
According to this method, since air blowing is effected in the state where the thick portions 97a, 97a are in contact with the molding die 93 and cooled, the extension of the thick portions 97a, 97a of the parison 97 is suppressed during the extension by blowing and almost only the thin portions 97b, 97b are extended. Therefore, by preliminarily forming the thin portions 97b, 97b to be slightly thicker in consideration of the allowance for the thickness reduction due to the extension, no extreme thickness variation such as performed finally in the resinous molding product is necessary.
By this molding method, such a defect that an undulation portion takes place on the surface of the parison 97 or the parison 97 is extruded in the bent state can be prevented and a molding product having a wide range of thickness variation ratio (thickness ratio) can be obtained. Furthermore, since each thick portion 97a of the parison 97 is secured at the thickness set at the extension process by the expander member 93, the thickness of the thick portion of the molding product can be set at a high accuracy.
By employment of the above-described method, even in the case where a parison is formed almost uniformly in thickness in the circumferential direction, namely, a parison is extruded downwards in the form of the so-called uniform thickness parison, a molding product in which a specific portion is formed thicker can be obtained. Namely, in this case, a parison can be extruded downwards without the possibility of defects such as the surface undulation or other deformations of the parison, and further, a wide range of thickness variation ratio can be given to the molding product.
Meanwhile, the above-described method is so arranged that, by bringing the expander members into contact with the specific portions (thick portions 97a) of the parison 97, the specific portions 97a are cooled and solidified to suppress the flow of the resinous material at these portion. In the state where the original thickness of said specific portions is retained as far as possible, portions (thin portions 97b) other than the specific portions 97a of the parison 97 are extended. The thickness difference between the specific portion 97a of the parison 97 and the other portion 97b caused by this extension determines the thickness variation ratio of the molding.
The extent to which the thickness difference between the specific portion 97a and the other portion 97b is secured depends largely upon the temperature difference between both portions 97a and 97b at the time of extension. Therefore, how to set the temperature difference is very much important for securing a predetermined thickness ratio on the molding.
Furthermore, in order to provide a predetermined thickness ratio on the molding, it is important to effectively cool and solidify the specific portion 97a.
However, when the specific portion 97a is cooled and solidified too excessively, the moldability of this portion at the blow-molding after extension is impaired, resulting in the probability of yielding defective moldings.
Furthermore, there has been such a problem that when the temperature at the surface of the specific portion 97a contacted by the expanding member 98 is too low and solidified excessively, both members become easy to stick to each other, which worsens the separability in separating the expanding member 98 from the contact portion of the specific portion 97a after completion of the extension process.
On the other hand, in obtaining moldings in which the specific portions are made thicker, if the range of thick thickness formation can be adjusted to increase or decrease, it is very convenient for designing and manufacturing various moldings having different requirements for the range of thick thickness formation.